Conventional nail guns have a well-known activation mechanism for initiating a nail-driving operation when two operations are performed simultaneously by the operator of the nail gun: (1) an operation to press a push lever protruding from the nail gun at a position near a nail ejection opening against a workpiece, such as a piece of wood, causing the push lever to slide inward along the axial direction of the nail gun body, and (2) an operation to pull a trigger on the nail gun. Some such nail guns disclosed in U.S. Pat. No. 5,551,620 have a mechanism for changing the operating mode of the nail gun in response to an operator selection.